The VoIP (Voice over Internet Protocol) is a technique for performing audio communication over an IP (Internet Protocol) network, and is widely applied e.g. to internet telephones and extension telephones using an interoffice LAN (local Area Network).
In the VoIP, basic call control of a session as a connection relationship between applications operated on the IP network (start, change and disconnection of the session) is carried out using a protocol called SIP (Session Initiation Protocol).
Further, a protocol called SDP (Session Description Protocol) is used for handling a session. SDP cooperates with SIP to perform session negotiation, and select an audio medium to execute audio communication.
FIG. 19 illustrates audio communication using VoIP. Communication apparatuses 51 and 52 both include an ITU-T G. 711 audio medium as an audio medium. The communication apparatuses 51 and 52 perform session negotiation using SDP, and thereafter performs audio communication with each other via the respective ITU-T G. 711 audio media.
Alternatively, if the communication apparatuses 51 and 52 both include an ITU-T G. 729 audio medium as an audio medium, the communication apparatuses 51 and 52 perform session negotiation using SDP, and thereafter perform audio communication with each other via the respective ITU-T G. 729 audio media.
Note that ITU-T G. 711 is an audio compression algorithm using a PCM (Pulse Code Modulation) audio codec (CODEC: encoding and decoding), and ITU-TG. 729 is an audio compression algorithm using a CS-ACELP (Conjugate Structure-Algebraic Code Excited Linear Prediction) audio codec.
As a conventional technique, there has been proposed a technique of a multimedia communication system in which both users connected through the VoIP communication system can select a communication medium independently of each other (Japanese Laid-open Patent Publication No. 2008-085837). Further, there has been proposed a technique of a VoIP gateway apparatus which automatically selects an optimum service out of a plurality of IP phone services to which a user subscribes (Japanese Laid-open Patent Publication No. 2005-252662).
In the VoIP audio communication, as described above, when the self apparatus and a counterpart apparatus both have a function of the same audio medium, it is possible to perform direct audio communication between them. However, if the both apparatuses do not have the same audio medium, the apparatuses perform communication via a gateway apparatus having a medium conversion function.
FIG. 20 illustrates VoIP audio communication via a gateway apparatus. It is assumed that the communication apparatus 51 includes the ITU-T G. 711 audio medium, and the communication apparatus 52 includes the ITU-T G. 729 audio medium.
Further, a gateway apparatus 5 includes both of the ITU-T G. 711 audio medium and the ITU-T G. 729 audio medium. That is, the gateway apparatus 53 has a function of interconversion between ITU-T G. 711 and ITU-T G. 729.
In general, a gateway apparatus is installed at a border between networks, and performs medium conversion of communication data when the communication data is transmitted across the border between the networks. The gateway apparatus on the VoIP network is realized e.g. by an SIP server.
When the communication apparatuses 51 and 52 perform communication with each other, the gateway apparatus 53 converts ITU-T G. 711 audio data transmitted from the communication apparatus 51 to ITU-T G. 729 audio data, and transmits the converted audio data to the communication apparatus 52. Further, the gateway apparatus 53 converts ITU-T G. 729 audio data transmitted from the communication apparatus 52 to ITU-T G. 711 audio data, and transmits the converted audio data to the communication apparatus 51.
FIG. 21 illustrates VoIP audio communication via gateway apparatuses. FIG. 21 illustrates a network state in which a plurality of gateway apparatuses are installed. A gateway apparatus 53-1 includes the ITU-T G. 711 audio medium and an AMR (adaptive multi rate) audio medium, and performs interconversion of audio data between ITU-T G. 711 and AMR.
A gateway apparatus 53-2 includes the ITU-T G. 729 medium and the AMR audio medium, and performs interconversion of audio data between ITU-T G. 729 and AMR. Note that AMR is an audio codec used in third-generation mobile communication, and is a standard system developed by 3GP (3rd generation partnership project).
When the communication apparatuses 51 and 52 perform communication with each other, the gateway apparatus 53-1 converts ITU-T G. 711 audio data transmitted from the communication apparatus 51 to AMR audio data, and transmits the converted audio data to the gateway apparatus 53-2. The gateway apparatus 53-2 converts the AMR audio data to ITU-T G. 729 audio data, and transmits the converted audio data to the communication apparatus 52.
Further, the gateway apparatus 53-2 converts ITU-T G. 729 audio data transmitted from the communication apparatus 52 to AMR audio data, and transmits the converted audio data to the gateway apparatus 53-1. The gateway apparatus 53-1 converts the AMR audio data to ITU-T G. 711 audio data, and transmits the converted audio data to the communication apparatus 51.
As described above, when the communication apparatuses 51 and 52 include the audio media different from each other, it is not possible to perform direct communication between the apparatuses, and hence communication is performed via the gateway apparatus(s) where medium conversion is performed.
However, in the conventional techniques, there has been no means for recognizing which type of an audio medium is provided in the gateway apparatus or the other end of communication, and further it has been impossible to recognize which gateway apparatus can provide a pass to be established for minimizing communication costs. This brings about a problem of a higher possibility of establishing a wasteful path routed through a plurality of gateway apparatuses, resulting in degraded communication efficiency.
FIG. 22 illustrates VoIP audio communication via gateway apparatuses. It is assumed here that the gateway apparatus 53-1 illustrated in FIG. 21 includes, as audio media, not only the ITU-T G. 711 audio medium and the AMR audio medium but also the ITU-T G. 729 audio medium (i.e. the gateway apparatus 53-1 has a function of interconverting these three audio media).
In this case, when the communication apparatuses 51 and 52 perform communication with each other, it is possible to perform communication only via the gateway apparatus 53-1 without being routed through the gateway apparatus 53-2. However, in the conventional techniques, it is impossible to recognize that the gateway apparatus 53-1 also includes the ITU-T G. 729 audio medium that the gateway apparatus 53-2 has, and hence there is a possibility that communication is performed by being unnecessarily routed through the two gateway apparatuses 53-1 and 53-2, causing degraded communication efficiency.
Further, particularly when communication is performed over a plurality of networks, since a plurality of gateway apparatuses are installed, communication data sometimes passes through a plurality of gateway apparatuses unnecessary for the data to pass through, which causes marked degradation of communication efficiency. Although the VoIP audio communication has been described, by way of example, the same problem can be caused in other communication protocols and communication media.